The Lost System. Even today, reports are still confused as to exactly what happened, but these things are known:

There was a small colony - less than 20,000, gathered in one town on the only barely habitable planet. Never self-sufficient, it had survived as a subsidized fuel depot and emergency repair station.

A small team of Nephilim attempted to set up a wormhole using Chandley's star. Had they succeeded and brought through reinforcements, they would have been able to run rampant over the Hawking and Argent sectors, and make significant incursions into the Enigma and Sol sectors before running into any real resistance.

This effort would take weeks or months - it is unclear exactly how long - and thus relied on stealth. Fortunately, it was discovered, and Confederation military alerted. (The Nephilim killed all the colonists, and then did a poor job of pretending to be them. Captains of passing ships were instructed to humor them until a military response could arrive. This ruse held.)

Six Yorktown-class carriers were the only capital ships able to intercept them in time, and only because they had been mothballed in what was thought to be the safe, uncontested side of Terran Confederation space - away from the Kilrathi Empire, previous Nephilim incursions, and other events. Fighters and ordinance were hastily manufactured or diverted while the carriers were prepared.

Destroying the Nephilim equipment turned Chandley's star into a black hole. (Who knew this would happen, and when they knew, is hotly debated - mostly by those who were not there.) Two of the carriers were either lost, or escaped to Destiny or Yeager. The other four escaped to Grissom.

While the jump points are far enough away that things coming through them are not immediately sucked into the black hole (the Nephilim did not add that much mass to the star, though there is enough gravity to affect navigation), the system is now flooded with radiation. Human crews in well-shielded ships that go there and immediately jump back accumulate a severe case of radiation poisoning from their brief exposure; going anywhere within the system before returning, or attempting to cross to another jump point, would be guaranteed suicide. Completely automated drones have not been able to reach the Grissom jump point from the White jump point or vice versa, save as radiation-fried husks with all electronics - including jump drive controls - destroyed (necessitating retrieval by other drones, jumping at precisely calculated times). Given that, and the fact that no such probes from Destiny or Yeager have come through, it is assumed that probes sent to Destiny and Yeager did not survive the trip.

Consolation is taken from the high likelihood that the same thing happened on the far side of the wormhole, which would have been a major Nephilim colony, with a population in the millions or billions.

As far as most people are concerned, "impassible" and "going there = suicide" sum up Chandley today.